1. Field of the Invention
This invention relates to a process for forming multilayer thin films, such as amorphous semiconductor superlattices.
2. Discussion of the Background
Development of amorphous semiconductor thin films is now in progress toward its commercial application to solar cells, thin film transistors or the like. But, from a performance aspect, the research works seem to have reached a plateau. The major reason is that the mobility of carriers is low in amorphous semiconductors which essentially contains many defects. As a new potential material which can break through the current situation, a great attention is paid to amorphous semiconductor superlattices.
The amorphous semiconductor superlattice is a structure consisting of alternating layers of two amorphous semiconductor materials which are different either in composition and band gap or in doped impurity, each layer having a thickness of several angstroms to several tens angstroms. For brevity of description, two different amorphous semiconductor layers are designated layers A and B, hereinafter. Amorphous semiconductor superlattice thin films are prepared in the prior art by plasma- or light-excited chemical vapor deposition (CVD). One commonly used method is by mechanically switching the supply of reactant gases to alternately deposit layers A and B. More particularly, a reactant gas from which layer A is deposited is first introduced into a reaction chamber where the reactant is excited and decomposed by light or plasma to thereby deposit layer A until the desired film thickness is reached. The feeding of reactant gas is stopped and the reaction chamber is evacuated. Thereafter, another reactant gas from which layer B is deposited is introduced into the reaction chamber where layer B is similarly deposited until the desired film thickness is reached. The reaction chamber is again evacuated to avoid the cross contamination at the bounding of two layers. A superlattice thin film is prepared by repeating this procedure the predetermined cycles. For example, a structure of alternating layers of a-Si (amorphous-Si) and a-SiC (amorphous-SiC) can be prepared by carrying out decomposition of a silicon-containing reactant to deposit a-Si and then carrying out decomposition of a mixture of a silicon-containing reactant and a carbon-containing reactant to deposit a-SiC. A superlattice thin film is obtained by repeating this procedure.
These prior art methods for forming superlattice thin films by plasma- or light-excited CVD, however, essentially require mechanical switch-over of reactant gases. Every time the reaction to form a film of one type is completed, the reaction chamber is purged of the reactant gas. The need for vacuum operation after every film formation results in inefficient production of a superlattice film.
In the above-described methods requiring switch-over of reactant gases, deposition of one layer is essentially one process of operation. Preparation of a superlattice thin film which is a multilayer laminate film thus requires a number of processes, presenting a bar against commercial application.
An alternative method uses a reaction chamber for forming layer A and another reaction chamber for forming layer B. The two chambers are juxtaposed through a partition. A rotor is provide in association with the partition such that the rotor moves at a predetermined speed back and forth between the two chambers while rotating. A substrate rests on the rotor. Then the substrate is moved into and out of the two chambers alternately with the rotation of the rotor whereby layers A and B are alternately formed on the substrate. This method, however, is also inefficient.